A ZSE RADAR ASSOCIATE TRAINEE INADVERTENTLY REROUTES A B737 TEST FLT TO SEA INSTEAD OF BFI.

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WHEN COMPLETE WITH TRAINING/'TEST FLYING' AN ACFT; B737 REQUESTED TO BE REROUTED BACK TO BFI (BOEING FIELD) VIA DIRECT YAKIMA AND THE CHINS 5 ARR. I INADVERTENTLY AMENDED THE RTE VIA THE CORRECT ROUTING; BUT TO THE INCORRECT ARPT; SEA (SEATTLE-TACOMA INTL). MY INSTRUCTOR; THE R-SIDE WORKING THE SECTOR; AND I FAILED TO CATCH THE ERROR BEFORE B737 LEFT OUR AIRSPACE. THE ERROR WAS NOT CAUGHT UNTIL THE ACFT WAS ON FINAL APCH INTO SEATAC. THERE WAS NO LOSS OF SEPARATION AND B737 WAS REROUTED TO ITS CORRECT DEST OF BFI. WHAT CAUSED THE PROB WAS MY INEXPERIENCE. THE TRUTH IS THAT I DO KNOW BETTER (AND DO KNOW THE DIFFERENCE BTWN THE 2 ARPTS); HOWEVER; THERE WAS (EVIDENTLY) SOME SORT OF CONFUSION IN THE PROCESS (OR PROCESSING) OF DOING THE REROUTE. ONE FACTOR THAT CONTRIBUTED TO MY 'CONFUSION' IS THAT THE 2 ARPTS ARE ABOUT 5 MI APART. THE SECOND FACTOR IS THAT I'M ONLY 3 1/2 MONTHS 'OLD' AS AN ATC SPECIALIST. I'VE CAUGHT MYSELF BEFORE WHEN DOING REROUTES; HOWEVER; THE NUMBER OF ERRORS I MADE AND HAD TO CORRECT HAD DWINDLED DRASTICALLY IN THE LAST MONTH TO 2 MONTHS. WITH THE AMOUNT OF INFO AND PROCESSING THAT I DO DAILY; IT'S NOT SURPRISING THAT THIS KIND OF MISTAKE HAPPENED. HOWEVER; IT STILL IS UNACCEPTABLE THAT IT DID. WHAT CAN BE DONE TO PREVENT THIS FROM HAPPENING AGAIN? GET MORE EXPERIENCE; BE VIGILANT; DON'T 'HURRY' TO DO THINGS; DOUBLE-CHK MY WORK; AND REMEMBER THAT PEOPLE'S LIVES ARE COUNTING ON THE WORK I DO; SO I BETTER DO IT CORRECTLY. IN THINKING AND IN PERSPECTIVE; REMEMBERING THAT LAST FACTOR HAS HELPED ME TO FOCUS MORE AND TO TRULY LEARN FROM THIS MISTAKE.